1. Field of the Invention
The present invention relates to a master cylinder provided bodily with a booster which augments the stepping force on a brake pedal of a vehicle.
2. Discussion of the Related Art
Heretofore, there has been known a master cylinder provided bodily with a negative-pressure operated booster which augments the stepping force on a brake pedal, as described in Japanese examined utility model No. 63-1635 (1988-1635). In the know device, a cylinder body is formed with a cylinder, which opens to a booster mounting surface of the cylinder body mounting a booster. A piston which has the rear end thereof entering a constant-pressure chamber of the booster and which is operated by an output shaft of the booster is inserted into a piston guide secured to the cylinder. A fluid-tight seal member inserted into the cylinder is secured to the piston for defining the front area as a fluid pressure chamber. The cylinder is formed with a drain hole, which makes the fluid pressure chamber communicate with a reservoir when the piston is retracted to an inoperative position, but opens to a drain area behind the fluid-tight seal member when the piston is advanced. The piston guide is formed with an internal bore through which a small diameter portion of the piston passes and is in close contact at a flange portion thereof with the end surface of an annular protruding portion of the cylinder body entering the constant-pressure chamber. The flange portion is formed with an internal seal stepped portion by enlarging an opening end portion of the internal bore and is also formed with an external seal stepped portion by diminishing the end portion of the external surface of the piston guide. The piston guide and the annular protruding portion are kept in close contact with each other and are covered by a cover member which is drawn toward the booster mounting surface as being engaged with a booster shell of the booster secured to the booster mounting surface. A slide seal member is fit in the internal seal stepped portion for sliding contact with an external surface of the piston thereby to fluid-tightly partition the drain portion from the constant pressure chamber, while a fluid-tight seal member is fit between the external seal stepped portion and the cover member thereby to fluid-tightly partition the drain portion from the constant-pressure chamber. At an engagement portion between the booster shell and the cover member, another seal member is interposed between both of the booster shell and the cover member and the booster mounting surface for partitioning the constant-pressure chamber from the atmosphere and also for partitioning the drain area from the atmosphere.
However, in the prior art master cylinder provided bodily with the booster, the piston guide and the annular protruding portion are closely contacted with each other and are covered by the cover member. Thus, the slide seal member and the fluid-tight seal member which are fit respectively in the internal and external seal stepped portions formed on the piston guide are to partition the drain area from the constant-pressure chamber. In addition, the seal member which is interposed between the both of the booster shell and the cover member and the booster mounting surface of the cylinder body is also to partition the constant-pressure chamber from the atmosphere as well as to partition the drain area from the atmosphere. That is, the prior art arrangement disadvantageously requires to use many seal elements and results not only in complication in configuration but also in an increase in manufacturing cost.